Conventional amplifiers and closed loop control amplifiers to drive valves, in particular proportional solenoid valves are made up of a number of individual components. Thus the current through the solenoid windings is controlled by an endstage to which power is supplied by a suitable power supply. The amplifier further receives signals defining a number of desired values such as valve piston positions, fluid flow volumes and pressures. Additionally desired values are set by potentiometers and will be supplied to the amplifier by operating switches. Desired values for the piston position of the valve or the height of the pressure for example are generated in an analogue manner. A main component of each amplifier is the control electronics which is connected between the desired value input and the endstage and which generates the positioning signals for the control and output characteristics of the valve in accordance with a valve specific program. This program will vary in response to the function of the valve and the valve size. As an example such control may be employed in directly operated or pilot operated directional proportional valves operating with and without feed-back, directly operated or pilot operated pressure valves, pressure limiting valves or pressure reducing valves, further flow valves, directional servo-valves or servo-pressure-valves. Such control amplifier may operate a variety of valves having a predetermined function which requires a certain algorithm for generating positioning signals in the amplifier to obtain a predetermined interrelationship between setting a desired value and generating a suitable output current in the endstage. In addition, the size of the valve and thus the stroke and the geometry of the valve piston must be considered to obtain the proper signal.
These requirements result in control electronics and amplifiers are valve specific. This means that each valve type or at least each valve family needs an individual specifically designed amplifier.
Moreover the control electronics may include further components all known to the artisan. For example to obtain a continuous transient of response from a sudden variation of a desired value a ramp circuit may be useful. Components for the recognition of failure, for example for indicating a broken cable may be helpful. As mentioned before, the amplifier may be a closed loop amplifier so that additional inputs are provided for the feed-back of actual values such as the solenoid current and/or the valve piston position or pressures occuring at the valve or adjusted by the valve. Still further the endstage is controlled by a pulse width modulated signal thus supplying a pulsed current having a fixed or freely selected frequency to the valve solenoid. The forgoing enumeration of individual components of a valve amplifier should not be considered to be complete. Further details are found in the handbooks of valve manufacturers, for example in MANNESMANN REXROTH RD 29003/3.88 "Hydraulic and electronic components for proportional and servo-systems".
The amplifier components are regularly mounted on a supporting plate which in turn may be mounted to suit a specific purpose. Simple circuit amplifiers having a limited number of terminals and lacking setting means such as desired value setters may be mounted in a casing which is called a module. More sophisticated amplifiers which include means for setting valve specific and amplifiers specific parameters such as desired values, ramp times etc. are mounted on a supporting plate including a front plate to accomodate switches, push buttons or display elements. A number of amplifiers of this type are combined in magazines. Modules and magazines are assembled in a switching board. Otherwise it is possible to mount an amplifier directly to a valve adjacent the valve casing or to secure it to a valve sandwich plate.
A substantial disadvantage of the prior art valve amplifiers is the need for a great variety of individual amplifiers to serve the various types of valves and valve sizes to be held on stock so that a particular amplifier which is associated to a particular valve is available. According to the prior art the individual valve types and valve sizes further need specific amplifiers in a variety of embodiments. For example one embodiment for each valve integrated assembly, the modul assembly or the magazine assembly must be stocked. All the various amplifier types must be separately manufactured and held-on stock.